Catalytic converter system for i.c.-engine with divided flow and two converters

ABSTRACT

An exhaust system for a multi-cylinder internal combustion engine has at least two exhaust valves per cylinder. A first exhaust valve in each cylinder is connected to a first catalyst, and a second exhaust valve in each cylinder is connected to a second catalyst which opens out upstream of the first catalyst. The first exhaust valves or their outlets are kept closed during a starting phase of the engine, so that, during the starting phase, all the exhaust gases first pass through the second catalyst which is smaller than the first catalyst and is positioned closer than the latter to the engine.

TECHNICAL FIELD

The invention relates to an exhaust system for a multi-cylinder internalcombustion engine.

STATE OF THE ART

Turbocharging an internal combustion engine, where supercharging iscarried out by means of an exhaust-driven turbocompressor, has manyadvantages from the point of view of engine technology, and turbochargedengines are used increasingly frequently in cars.

One problem with modern turbocharged engines, however, is that it takesa relatively long time after starting to achieve good efficiency of acatalyst located downstream of the exhaust gas turbine. One reason forthis is that the exhaust-gas turbine causes a drop in temperature whencold-starting. Furthermore, the narrow inlet at the turbine wheelresults in high flow rates which lead to a high heat-transmissioncoefficient. The overall result is great thermal inertia upstream of thecatalyst and, consequently, delayed ignition of the catalyst. Moreover,positioning the catalyst downstream of the exhaust-gas turbine leads toa relatively great distance between the engine and the catalyst. Reasonsof space also make it usual to position the catalyst a long way from theengine, often under the floor of the vehicle.

OBJECTS OF THE INVENTION

The object of the invention is to produce an improved internalcombustion engine from the point of view of exhaust gas. Another objectis to achieve effective emission control more rapidly than previouslywhen cold-starting.

DESCRIPTION OF THE INVENTION

These objects of the invention are achieved by means of an internalexhaust system in accordance with the invention.

By using a number of exhaust valves in each cylinder for distributingthe exhaust-gas flow, it is possible to position a relatively smallcatalyst close to the engine and, during a starting phase, to guide allthe exhaust gases to this catalyst before they reach an ordinarycatalyst positioned downstream. The small catalyst starts functioningrapidly and, when the engine is hot, is exposed to only a proportion ofthe exhaust gases of the engine.

The solution according to the invention is particularly advantageous forturbo-engines because it means that exhaust gases do not have to be fedto the exhaust-gas turbine in connection with starting. As a result, thedelay in effective emission control, which otherwise occurs inturbo-engines, is avoided.

The arrangement with a divided exhaust period also makes it possible toscavenge the engine more effectively than previously, with improvedengine performance as a result.

Further features and advantages of the solution according to theinvention emerge from the description and the other patent claims.

The invention is described below in greater detail with reference toexemplary embodiments shown in the drawing.

DESCRIPTION OF THE FIGURES

In the drawing:

FIG. 1 shows diagrammatically an exhaust system according to theinvention,

FIG. 2 shows an exhaust system according to FIG. 1 in a turbochargedengine,

FIG. 3 shows the valve diagram for the exhaust valves in an engineaccording to FIG. 2 in the cold state,

FIG. 4 corresponds to FIG. 3 but shows a hot engine instead, and

FIG. 5 shows an alternative embodiment of a detail of FIGS. 1 and 2.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows diagrammatically an exhaust system according to theinvention in a multi-cylinder internal combustion engine 1. Thecylinders of the engine each have at least two exhaust valves 2 and 3.From the first exhaust valves 2 of the cylinders, exhaust gas is led outto a first exhaust manifold 4 common to the cylinders, which isconnected to a first catalyst 10 via a first exhaust pipe 11. From thesecond exhaust valves 3 of the cylinders, exhaust gas if led out to asecond exhaust manifold 5 common to the cylinders, which is connected tothe first catalyst 10 via a second exhaust pipe 12 in which there is asecond catalyst 13. One or more silencer(s) (not shown) is or arelocated downstream of the first catalyst 10 in the conventional manner.

FIG. 2 shows how the exhaust system in FIG. 1 is used for aturbo-engine, that is to say an engine in which supercharging is carriedout by means of a compressor driven via an exhaust-driven turbine. Theexhaust-gas turbine 14 is in this case connected into the exhaust pipe11 and is therefore fed from the first exhaust manifold 4 and the firstexhaust valves 2. A compressor 15 driven by the exhaust-gas turbine 14provides the engine with charge air which is here cooled in anintercooler 16 before it is fed into the engine in the conventionalmanner (not shown in greater detail).

When the engine 1 is started, and as long as it is still cold, the firstvalves 2 are kept closed and all exhaust-gas discharge is effected viathe second valves 3 which feed all the exhaust gases to the secondcatalyst 13 via the second exhaust manifold 5. In connection withstarting, peak power is not required from the engine, for which reasonthe second catalyst 13 can be dimensioned for a relatively smallexhaust-gas flow and be positioned close or very close to the engine. Inthis way, it can be heated rapidly and achieve good functioning, so thateffective emission control is achieved very quickly on starting. Duringthe time that the second catalyst 13 is working, the first catalyst 10is heated. It is normally serviceable after a period of time of roughly30 s from starting. As the first catalyst 10 has thus become hot, thefirst exhaust valves 2 are set in operation so that they can feedexhaust gases out to the first exhaust manifold 4 and on from there tothe first catalyst 10. A certain proportion of the exhaust-gas flow cancontinue to pass through the second catalyst 13 on the way to the firstcatalyst 10. If appropriate, a shunt valve 20 can be provided in theexhaust pipe 12, which valve can be adjusted when the engine is hot sothat the second catalyst 13 is bypassed via a pipe 21 (FIG. 5).

In the example according to FIG. 2, the exhaust-gas energy is used tooperate an exhaust-gas turbine 14, but the functioning is in otherrespects the same as for the exhaust system in FIG. 1.

The control of the exhaust valves in each cylinder can be effected in anumber of different ways in order to achieve the desired function andthe desired ratio between different exhaust-gas flows. A disadvantage ofkeeping the first exhaust valves closed for a period is that the enginepower is limited during this period. This disadvantage is usually small,however, in comparison with the gain in terms of improved emissioncontrol.

A control of the exhaust valves 2 and 3 suitable for the embodimentaccording to FIG. 2 is shown in FIG. 3, with the engine cold, and FIG.4, with the engine hot. In each figure, the left diagram a) representsthe first exhaust valve 2, while the right diagram b) represents thesecond exhaust valve 3. In each diagram, A indicates the top dead centreof the piston, B the bottom dead centre of the piston, C the openingposition of the exhaust valve, D the closing position of the exhaustvalve and E the open time of the exhaust valve.

As can be seen according to FIG. 3, when the engine is cold, the firstexhaust valve 2 (FIG. 3a) is kept closed, while the second exhaust valve3 (FIG. 3b) opens at C, roughly 30° before the bottom dead centre B andis kept open until D, roughly 15° after the top dead centre A. Allexhaust-gas discharge is thus effected via the second exhaust valve 3during a long open time E, roughly 225°.

When the engine is hot, according to FIG. 4, the first exhaust valve 2(FIG. 4a) starts functioning and opens at C, roughly 45° before thebottom dead centre B, and is kept open until D, roughly 30° before thetop dead centre A. The open time E is thus roughly 195° in this case,but is dependent on the number of cylinders connected to the exhaustmanifold. The angular values indicated are only an example for afour-cylinder engine. The second exhaust valve 3 (FIG. 4b) now opensslightly later than previously, at C in fact, roughly 30° after thebottom dead centre B, and is kept open until the same point aspreviously, namely until D, roughly 15° after the top dead centre A.This division of the open time between the valves 2 and 3 provideseffective scavenging of the engine.

A special advantage of the embodiment according to FIG. 2 is that themajor drop in temperature caused by the cold turbine 14 is avoided onstarting, and a rapidly activated second catalyst 13 can be usedinitially while the ordinary, first catalyst 10 is heated. The divisionof the exhaust stroke also has other advantages, such as, for example,improved emptying of the cylinder because low-pressure exhaust gases donot have to pass through the turbine but can be conducted past thelatter.

In order to be capable of handling an adequately large exhaust-gas flowwhen the engine is cold, it is appropriate if the second exhaust valvecan open more, that is to say have a greater lifting movement, when theengine is cold than when the engine is hot.

As the majority of the exhaust-gas passes through the main catalyst athigh power levels, the second catalyst 13 can suitably be designed so asto be capable of handling roughly 20-25% of the maximum exhaust-gas flowat full load of the engine, but should still be positioned as close tothe engine as possible in order to undergo rapid heating.

Instead of keeping the exhaust valves 2 closed for a time afterstarting, it would also be possible to close their outlet, that is tosay the first exhaust pipe 11, by means of a valve which can be openedand closed in order to achieve the desired distribution of theexhaust-gas flow. Examples of such an arrangement are shown in FIGS. 1and 2, in which a valve 30 has been mounted in the first exhaust pipe11. In the arrangement in FIG. 2, the valve 30 can suitably be mountedupstream of the exhaust-gas turbine 14, but positioning 30′ downstreamof the exhaust-gas turbine 14 is also possible. The valve 30 shown canadvantageously be electrically controlled and will of course be in theopen position during the major part of the operation of the engine.

What is claimed is:
 1. Exhaust system for a multi-cylinder internalcombustion engine comprising: a first larger catalyst connected to theengine, and a second, smaller catalyst also connected to the engine,said second catalyst discharging into an exhaust pipe upstream of thefirst catalyst so that, during a starting phase of the engine, all theexhaust gases pass through the second catalyst before they reach thefirst catalyst, at least one first exhaust valve in each cylinder isconnected to a first exhaust manifold, which in turn is connected to thefirst catalyst via an exhaust-gas turbine in a turbo compressor forsupercharging the engine, at least one second exhaust valve in eachcylinder is connected to a second exhaust manifold, which is connectedto the second catalyst, that opens into the exhaust pipe between theturbine outlet and the first catalyst, the first exhaust valves arearranged to be closed during the starting phase.
 2. Exhaust systemaccording to claim 1, characterized in that the first exhaust valve isarranged so as to be open only when the engine is hot, when the pistonis at its bottom dead center, the valve opening before and closing afterthe bottom dead center so as to deliver high-pressure gas to theexhaust-gas turbine.
 3. Exhaust system according to claim 1,characterized in that the second exhaust valve is arranged so as, whenthe engine is hot, to open after the first exhaust valve.
 4. Exhaustsystem according to claim 1, characterized in that the second exhaustvalve is arranged so as, during the starting phase, when the engine iscold, to open before the piston has reached its bottom dead center. 5.Exhaust system according to claim 1, characterized in that there arearranged upstream of the second catalyst a shunt valve and, associatedwith the latter, a shunt pipe which opens out upstream of the firstcatalyst, and in that the shunt valve is arranged so as, when the engineis hot, to shunt the exhaust gases past the second catalyst.
 6. Exhaustsystem according to claim 1, characterized in that the second catalystis arranged adjacent to the second exhaust manifold.
 7. Exhaust systemfor a multi-cylinder internal combustion engine comprising a firstlarger catalyst connected to the engine, and a second, smaller catalystalso connected to the engine, said second catalyst discharging into anexhaust pipe upstream of the first catalyst so that, during a startingphase of the engine, all the exhaust gases pass through the secondcatalyst before they reach the first catalyst, at least one firstexhaust valve in each cylinder is connected to a first exhaust manifold,which in turn is connected to the first catalyst via an exhaust-gasturbine in a turbo compressor for supercharging the engine, at least onesecond exhaust valve in each cylinder is connected to a second exhaustmanifold, which is connected to the second catalyst, that opens into theexhaust pipe between the turbine outlet and the first catalyst, the atleast one first exhaust valves are arranged to be closed during thestarting phase, the at least one second exhaust valves are arranged soas to open more during the starting phase when the engine is cold thanwhen the engine is hot.
 8. An exhaust system for an internal combustionengine including a plurality of cylinders, each of which has first andsecond valves, the exhaust system comprising: a first manifold connectedto the first valve of each of the cylinders for providing gas flowduring a startup and subsequent operation of the engine; a secondmanifold connected to the second valve of each of the cylinders and inflow isolation with the first manifold for providing gas flow during thesubsequent operation of the engine; a first catalyst incorporated in thefirst manifold and located downstream from the first valves forpurification of the exhaust gases discharged through the first valvesduring the startup and subsequent operations; a second catalystincorporated in the first manifold downstream from the first catalyst toprovide further purification of the exhaust gases exited from the firstcatalyst, the second catalyst being incorporated in the second manifoldto purify the exhaust gases discharged from the second valves during thesubsequent operation of the engine; a turbo-compressor incorporated inthe second manifold between the second valves and the second catalystfor producing secondary air for the engine during the subsequentoperation of the engine; and a controller for controlling the first andsecond valves so that the second valves are operated to block the gasflow along the second manifold during the startup operation and are opento allow the gas flow through the second manifold toward theturbo-compressor and the second catalyst during the subsequentoperation.